feat: add claude_args passthrough and structured_output (closes #1)

Bring the upstream v1 `claude_args` input and `structured_output` output
into this Gitea fork so downstream workflows can run the agent with
arbitrary Claude CLI flags (notably `--json-schema`) and read the
schema-validated verdict back as an action output, without an
agent-written file.

Approach (issue Option B, ported rather than bumped): the production
"Run Claude Code" step previously delegated to the external
anthropics/claude-code-base-action@v0.0.63, which has neither input.
Upstream base-action main has them but has migrated to the Claude Agent
SDK — a wholesale divergence that would change v0.0.63 behavior every
downstream step relies on. Instead this invokes the vendored base-action
directly via `bun run ${github.action_path}/base-action/src/index.ts`
(the same github.action_path pattern prepare.ts already uses, reliable
on Gitea), and ports only the two features onto the v0.0.63 process-spawn
engine.

- base-action/src/run-claude.ts: tokenize claude_args with shell-quote
  (comment-line stripping; quoted/file-path schemas survive intact) and
  append after the byte-identical BASE_ARGS, so empty claude_args yields
  an unchanged arg list; extract structured_output from the stream-json
  result event when --json-schema is present, failing loudly if absent.
- base-action/src/index.ts: forward INPUT_CLAUDE_ARGS.
- base-action/action.yml: add claude_args input + structured_output output.
- action.yml: add claude_args input + structured_output output; replace
  the external base-action delegation with a direct bun-run of the
  vendored copy (full INPUT_*/provider env contract derived from the
  vendored validate-env/index); bump the default Claude CLI install to
  2.1.160 (--json-schema requires a newer CLI than 1.0.117).
- shell-quote added to both package.json files; root bun.lock reconciled
  (it was stale vs package.json — non-frozen installs already resolved
  the newer tree).
- Tests for tokenizer, hasJsonSchema, and structured-output extraction.
- README: inputs/outputs tables, "Structured output" + "Version pins".
- examples/gitea-structured-output.yml.

Follow-up (cannot be done from this repo): consumers using a pre-baked
runner image (path_to_claude_code_executable) must rebuild that image
with a --json-schema-capable Claude CLI and restart the runner.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

base-action/package.json

@@ -10,11 +10,13 @@
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
-    "@actions/core": "^1.10.1"
+    "@actions/core": "^1.10.1",
+    "shell-quote": "^1.8.1"
   },
   "devDependencies": {
     "@types/bun": "^1.2.12",
     "@types/node": "^20.0.0",
+    "@types/shell-quote": "^1.7.5",
     "prettier": "3.5.3",
     "typescript": "^5.8.3"
   }